1. Field of the Invention
The present invention relates to a semiconductor device fabricated by using an SOI (Silicon on Insulator) substrate and a method of fabricating the same. Specifically, the invention relates to a semiconductor device including a thin film transistor (hereinafter referred to as TFT) formed on an SOI substrate.
Incidentally, in the present specification, the semiconductor device indicates any device capable of functioning by using semiconductor characteristics. Thus, the semiconductor device includes not only a TFT but also an electro-optical device typified by a liquid crystal display device or a photoelectric conversion device, a semiconductor circuit in which TFTs are integrated, and an electronic equipment containing such an electro-optical device or a semiconductor circuit as a part.
2. Description of the Related Art
In recent years, VLSI techniques have been remarkably developed, and attention has been paid to an SOI (Silicon on Insulator) structure for realizing low power consumption. This technique is such a technique that an active region (channel formation region) of an FET, which has been conventionally formed of bulk single crystal silicon, is made thin film single crystal silicon.
In an SOI substrate, a buried oxide film made of silicon oxide exists on single crystal silicon, and a single crystal silicon thin film is formed thereon. Various methods of fabricating such SOI substrates are known. As a typical SOI substrate, an SIMOX substrate is known. The term SIMOX is an abbreviation for Separation-by-Implanted Oxygen, and oxygen is ion implanted into a single crystal silicon substrate to form a buried oxide layer. The details of the SIMOX substrate are disclosed in [K. Izumi, M. Docken and H. Ariyoshi: “C.M.O.S. devices fabrication on buried SiO2 layers formed by oxygen implantation into silicon”, Electron. Lett., 14, 593-594 (1978)].
Recently, attention has also been paid to a bonded SOI substrate. The bonded SOI substrate realizes the SO structure by bonding two silicon substrates as suggested by its name. If this technique is used, a single crystal silicon thin film can be formed also on a ceramic substrate or the like.
Among the bonded SOI substrates, in recent years, attention has been especially paid to a technique called ELTRAN (registered trademark by Canon K.K.). This technique is a method of fabricating an SO substrate using selective etching of a porous silicon layer. The particular technique of the ELTRAN method is disclosed in, K. Sakaguchi et al., “Current Progress in Epitaxial Layer Transfer (ELTRAN)”, IEICE TRANS. ELECTRON. Vol. E80 C. No. 3 pp. 378-387 March 1997, in detail.
As another SQI technique attracting attention, there is a technique called Smart-Cut (registered trademark of SOITEC Co.). The Smart-Cut method is a technique developed by SOITEC Co. in France in 1996, and is a method of fabricating a bonded SoI substrate using hydrogen embrittlement. The particular technique of the Smart-Cut method is disclosed in “Industrial Research Society (Kogyo Chosa Kai); Electronic Material, August, pp. 83-87, 1977” in detail.
When the foregoing SOI substrate is fabricated, a single crystal silicon substrate having a main surface of a crystal face of a {100} plane (crystal orientation is <100> orientation) has been used in any technique. The reason is that the {100} plane has lowest interface state density (Qss) and is suitable for a field effect transistor that is sensitive to interface characteristics.
However, with respect to the SOI substrate used for a TFT, since a single crystal silicon thin film must be formed on an insulating layer, higher priority must be given to adhesion to the insulating layer than the interface state density. That is, even if the interface state density is low, it is meaningless if the single crystal silicon thin film peels off.